There is an increasing demand for anti-ballistic articles which provide greater performance per given areal density. For example, the increasing amount of electronic surveillance equipment which is mounted upon military helmets has lead to the problem of heavy and cumbersome helmets and consequently a desire for lighter weight helmets which deliver the same anti-ballistic performance. Further, to provide enhanced ballistic protection to vehicles there has been a need to increase the thickness of the anti-ballistic layers to a point where the anti-ballistic layering is beginning to compromise the maneuverability and handling of the vehicle. In addition to increased anti-ballistic performance, the growing demand for anti-ballistic articles has also lead to the need for shorter production cycle times to enable increased production capacity.
Anti-ballistic articles may also be frequently exposed to high temperatures for long durations. It is important that anti-ballistic articles have sufficient dimensional stability to maintain their shape, such that their functional performance is not compromised. One example is anti-ballistic panels positioned adjacent to an automotive engine. Anti-ballistic panels for vehicles are necessarily thick to achieve the required anti-ballistic performance and even small dimensional changes in the anti-ballistic panels may result in the panels exerting pressures on the vehicle's framework, thus compromising the vehicle's structural integrity. Another example is helmets and vests subject to a hot environment (e.g. in a vehicle's trunk). Thus, anti-ballistic articles designed for such personal protection which are subjected to a hot environment should also have good dimensional stability to ensure that they retain a comfortable fit against the wearer's body.
An object of the present invention is to provide an anti-ballistic article and a process for the manufacture of an anti-ballistic article which overcomes at least some of the abovementioned problems.
This object is achieved with a process for the manufacture of an anti-ballistic article comprising the steps of                a. forming a stack of sheets (“a stack”) by stacking 2 or more sheets, each sheet comprising one or more mono-layers of anti-ballistic fibers and optionally a thermoplastic binder, followed by        b. subjecting the stack of sheets to a reduced atmospheric pressure environment; and        c. while maintaining the reduced atmospheric pressure environment, subjecting said stack of sheets to a pressure of at least 10 MPa at an elevated temperature.        
It has been surprising found that the application of a reduced atmospheric pressure environment to the stack whilst the stack is compressed at an elevated temperature produces a surprising increase in anti-ballistic performance, as measure by the specific energy absorption (SEA) of the material. Preferably, the sheets are unidirectional sheets whereby in the stack, the direction of the anti-ballistic fibers in a monolayer is at an angle α to the fiber direction in an adjacent mono-layer. However, other assemblies and orientations of fibers may also be employed. For example, in another embodiment the sheet is a woven sheet.
The present process solves the problem of providing improved anti-ballistic performance.
The present invention provides several additional advantages derived from the resultant increase in SEA of the stack:
                1. An anti-ballistic article with increased anti-ballistic performance relative to conventionally produced anti-ballistic articles may be produced having the same areal density; and        2. A lighter more compact anti-ballistic article may be produced with the same anti-ballistic performance as a stack produced via conventional techniques.        